摘要：Context. Asteroseismology of massive pulsating stars of β Cep and SPB types can
help us to uncover the internal structure of massive stars and understand certain physical
phenomena that are taking place in their interiors. We study β Centauri (Agena), a
triple system with two massive fast-rotating early B-type components which show
p- and
g-mode
pulsations; the system’s secondary is also known to have a measurable magnetic field.
Aims. This paper aims to precisely determine the masses and detect
pulsation modes in the two massive components of β Cen with
BRITE-Constellation photometry. In addition, seismic models for the components are
considered and the effects of fast rotation are discussed. This is done to test the
limitations of seismic modeling for this very difficult case.
Methods. A simultaneous fit of visual and spectroscopic orbits is used
to self-consistently derive the orbital parameters, and subsequently the masses, of the
components. Time-series analysis of BRITE-Constellation data is used to detect pulsation
modes and derive their frequencies, amplitudes, phases, and rates of frequency change.
Theoretically-predicted frequencies are calculated for the appropriate evolutionary models
and their stability is checked. The effects of rotational splitting and coupling are also
presented.
Results. The derived masses of the two massive components are equal to
12.02 ± 0.13 and
10.58 ± 0.18
M⊙. The parameters of the wider, A–B
system, presently approaching periastron passage, are constrained. Analysis of the
combined blue- and red-filter BRITE-Constellation photometric data of the system revealed
the presence of 19 periodic terms, of which eight are likely g modes, nine are
p modes,
and the remaining two are combination terms. It cannot be excluded that one or two
low-frequency terms are rotational frequencies. It is possible that both components of
β Cen are
β Cep/SPB
hybrids. An attempt to use the apparent changes of frequency to distinguish which modes
originate in which component did not succeed, but there is potential for using this method
when more BRITE data become available.
Conclusions. Agena seems to be one of very few rapidly rotating massive
objects with rich p- and g-mode spectra, and precisely known masses. It can
therefore be used to gain a better understanding of the excitation of pulsations in
relatively rapidly rotating stars and their seismic modeling. Lacking proper mode
identification, the pulsation frequencies found in β Cen cannot yet be used to
constrain the internal structure of the components, but it may be possible to achieve this
in the future with the use of spectroscopy and spectropolarimetry. In particular, these
kinds of data can be used for mode identification since they provide new radial
velocities. In consequence, they may help to improve the orbital solution, derive more
precise masses, magnetic field strength and geometry, inclination angles, and reveal
rotation periods. They may also help to assign pulsation frequencies to components.
Finally, the case studied here illustrates the potential of BRITE-Constellation data for
the detection of rich-frequency spectra of small-amplitude modes in massive pulsating
stars.
关键词：stars: individual:βCentauri;stars:
oscillations;binaries: spectroscopic;binaries: visual;stars:
fundamental parameters
